Industrial controllers are special purpose computers used for controlling industrial processes in manufacturing equipment.
Under the direction of a stored program, the industrial controller examines a series of inputs, reflecting the status of the controlled process, and changes outputs effecting control of the industrial process. The inputs and outputs are most simply binary, that is on or off; however analog inputs and outputs assuming a value within a continuous range of values are also available.
In contrast to a computer, normally an industrial controller is "customized" to the particular process it is controlling, both by writing new control software that may be stored in the controller's memory, and by changing the hardware configuration of the controller to match the control task. The ability to change the hardware configuration is provided economically by dividing the industrial controller into a number of modules, each performing a different function. The modules may include a processor, a power supply, a communications port, and one or more input or output interface modules. The modules needed for a particular control task may be selectively linked together on a common backplane within a rack. In a typical hardware modification, additional input or output interface modules may be added to the rack so as to permit the control of additional or different kinds of equipment.
The input or output interface modules provide a means of converting between high power signals from or to the industrial process and low power digital signals that may be communicated along the backplane of the rack to the processor. For example, an "AC output" module may provide for the control of AC signals through a set of internal triacs which are controlled by low voltage digital signals received via the rack's backplane from the processor module. The triacs, or other similar power devices used in such input and output interface modules, generate heat as a result of their small but finite resistance in the on state and during the switching between off and on states. Ordinarily this heat is dissipated into the air surrounding the module and carried off by convection as the air moves through the open frame of the rack.
With a general reduction in the size of electronic circuitry and the development of compact industrial controllers, the ability to dissipate heat generated by the modules has been much reduced. The smaller size of the rack and modules both reduces the air flow through the industrial controller. Further, the small size of the rack increases the possibility of detrimental heat flow from one module to its neighbors. Because it is intended that the modules be freely mixed within a rack, such inter-module conduction places significant limits on the amount of heat that may be dissipated to the confines of the rack.
It is well known to provide for more heat dissipation by the use of forced air from fans or the like. Nevertheless, such a modification to the entire rack, by incorporating a fan and air conduits, is undesirable as it adversely and unnecessarily increases the cost of simple controller configurations which may not employ modules requiring high heat dissipation. Further, the need for a special rack limits the use of high heat dissipation modules in preexisting compact racks. Preferably, increased heat dissipation capability should be provided only when required, in a manner that minimizes the increase in cost to the basic system, and in a manner that is compatible with an installed base of systems and racks already in existence.